Studies of the molecular biology, structure and function of olfactory systems throughout the animal kingdom reveal many common design features (for example: large set of odorant receptors, precise axonal convergence, oscillatory synchronization of activity) across evolutionary distant animal species such as insects and primates (including humans). Interestingly, many of these design features are implemented differently (for example: different gene sequences, synaptic types and cell morphologies) across these many species: comparing these different implementations and, through this comparison, identifying their common high-level features helps us identify the rules of function of olfactory systems. Because olfactory circuits are tightly associated with memory structures-the memories of smells are particularly vivid and long lasting in humans-understanding olfactory coding also helps us better understand the nature of associative memories and their underlying biology. The present work is geared towards understanding the basic physiology of these olfactory/memory circuits. This work will constitute the electrophysiological foundation on which new cellular and systems studies of olfactory learning can be built in a particularly advantageous model system: experiments will be carried out with the brain of the fruit-fly Drosophifa, taking advantage of a century of genetics and molecular work by hundreds of laboratories on this system: by virtue of their accessibility and knowledge we now have of them, Drosophila genes can be controlled so as to either mark or manipulate specific neurons and circuits. By combining these powerful tools with electrode recordings and fast brain imaging tools, we can examine the role of specific molecules, neurons or synapses for olfactory function. The relatively small size of the Drosophila brain (~100,000 neurons) will also greatly simplify the task of deciphering the nature of neural codes in more complex systems, such as the human brain, thus contributing to a better understanding of the possible causes of perceptual disorders.